Very nice design with lots of energy providing a good alternative to the IMHO by now somewhat dated ft rechargeable battery pack.
Also works very well with protected cells if you're willing to solder wires to the cells. Just leave out the "pcb" STLs and make sure you rotate the cells in a way that the thin connection from top to bottom for the protection circuit on every cell has enough room (e.g in the 2 o'clock (left cell) and 10 o'clock (right cell) positions in the first picture) as otherwise the top and bottom halves might not latch well enough to stay together for long.
As I was a bit afraid of my two sons' exploratory urge I did not want to use cells with outside protection. I did experiment a bit with different battery connectors but ended up just soldering wires directly to the cells which worked without any issues using some flux (it probably helps to use a soldering station with a bit of power though).
I also tested that everything still works after soldering and assembling by connecting an electronic load (it provided 2-3 A without any issues with the cells in the pictures) and by shorting it (using the 20A position of my meter). I ended up using standard 0.14 mm² wires as they're so short (the middle one is for balancing only) that there is no noteworthy resistance even with the relatively thin gauge.